Optical disk drives employ a multi-element detector for generating a so-called track error signal (TES) that in other servo positioning systems is termed a position error signal. TES indicates relative position of a focused laser beam with respect to a center of a track, in most present day optical disks such track is a spiral groove formed in a disk substrate. One detector for generating TES is a far field detector that has two photo responsive elements. A line between the two elements is aligned tangentially with the spiral track (groove) and is preferably centered on the track center line. Such centering provides an accurate TES. The far field detector elements supply their respective signals to a differential amplifier that outputs a differential signal termed TES. TES can also be on two lines, one line connected to each of the elements. The signal amplitude difference of the signals on the two lines are a push-pull position error signal.
Tolerances for positioning a detector in an optical disk drive are relative to the size of the reflected laser beam being detected. In so-called near field (such as astigmatic detectors) detector the reflected laser beam is focussed to a small cross-sectional area making positional tolerances small. In contrast, a far field detector the reflected laser beam is focussed to have a larger cross-sectional area such that positioning tolerances for the detector are relatively large, such as an order of magnitude greater than in near field detectors. Such increased tolerances reduces sensitivity of the detector output signal, such as a track error signal, to thermal changes.
Also, to obtain a small size optical subassembly or head for an optical device, it is desired to use detectors that occupy small areas. This desire dictates that the photo element size in a detector have a smaller area than the cross-section of a reflected defocussed laser beam.
A problem in precise positioning of the focussed laser beam on a micron wide optical data-storing track or in precise optical track seeking operations arises from undesired surface contamination or perturbations that change reflectivity of the disk surface. Such disk surface perturbations can be in the millimeter and sub-millimeter size and yet cause sufficient noise in a tracking error signal (TES) to reduce disk drive performance. Such undesired changes in reflectivity (i.e. reduced reflectivity) have been found to cause a pair of areas of reduced reflected light intensity (shadows) in a far field light pattern of a reflected laser beam. Such far field light is used for optical disk tracking and seeking. It is believed that similar problems arise in near field light pattern of a reflected laser beam. One of the "shadows" is generated as the contaminant blocks or partially blocks some of the laser beam as the beam impinges on the contaminant as the laser beam enters the disk. A second "shadow" is generated by the reflected light beam traveling from the recording surface exiting the disk to return to the objective lens and the known detectors. The locations of the shadows remain balanced in the reflected laser beam as it travels through the objective lens into the optical system for detection. The shadows are centered about the optical axis of the reflected light beam. It has been discovered that if the far field detector that generates TES is not centered on the reflected light beam in a direction tangential to the optical track (groove) on the disk, then the shadows are imposed on the two photoelements of the far field detector asymmetrically or resulting in TES erroneous amplitudes. It is desired to overcome the above stated problem in a simple and effective manner.